In cellular mobile communication systems, a radio coverage area is implemented with different cell sizes and often even with multiple cell layers. A cell with a very small coverage area, i.e. a microcell, is used for instance in areas which are difficult to cover or which require a great deal of subscriber capacity. A large coverage area is implemented with a macrocell, which often completely covers the geographical area of one or more microcells. In a multilayer radio coverage, macrocells represent coverage areas of a higher level and microcells those of a lower level. The mobile station may move from one cell to another and register with a new base station. When the mobile station has a call in progress, switching the call (handover) to the new cell is also involved.
FIG. 1 of the accompanying drawings shows an example of a two-layer radio coverage in a mobile communication system. The simplified cell areas in FIG. 1 illustrate the areas in which the base station of each cell can offer a sufficient signal level to the mobile station. In FIG. 1, a large macrocell M covers the areas of smaller microcells A, B and C. A mobile station located in position x in cell A, for example, can in principle be served by the base station of either the macrocell M or the microcell A.
In radio networks where multilayer radio coverage is employed, it is advantageous to utilize the possibilities offered by the different cell layers by commanding mobile stations, on the basis of a certain classification, to register with the most suitable base station, avoiding however unnecessary handovers. One such classification criterion is the speed of a mobile station in a mobile communication network with one or more microcells whose coverage areas are located entirely within another cell, typically a macrocell. It is known that slow moving mobile stations are advantageously handed over to a microcell base station to prevent them from unnecessarily loading the base station of the macrocell. On the other hand, it is advantageous to hand fast moving mobile stations over to the base station of the macrocell to prevent frequently recurring handovers and even to prevent calls from being lost when mobile stations move fast outside the coverage area of a microcell.
In the two-layer radio coverage illustrated in FIG. 1, fast moving mobile stations are kept under the base station of the macrocell M in order to prevent unnecessary handovers. Only the mobile stations defined as slow moving are handed over to the microcell (A, B or C) in whose area the mobile station is located.
FIG. 2 of the appended drawings is a block diagram illustrating the structure of the GSM mobile communication system by way of example in a simplified manner. The units shown in FIG. 2 are also included in other mobile communication networks, although they may have different names. A mobile station MS that is in standby or active mode is constantly registered with a base transceiver station BTS. A base station system BSS consists of a base station controller BSC and base transceiver stations BTS under its control. A plurality of base station controllers BSC usually operate under a mobile services switching centre MSC. An MSC transmits messages to other MSCs.
A mobile station MS continuously measures signals from the base stations BTS that are closest to its location for a possible handover. In the GSM mobile communication network, for example, an MS may simultaneously measure the signal level and/or quality of both the serving base station and up to 32 other base stations. The serving base station informs the mobile station of the neighbouring cells it should measure. The measurement results of each cell are identified on the basis of a base station identity code BSIC.
The mobile station MS sends the measuring results regularly as a report message through the serving base station BTS to the base station controller BSC. A report message contains the measurement results of the serving base station and up to six best neighbouring base stations. The base station controller BSC makes the decisions relating to handover. If the target cell is controlled by another BSC, the handover is performed under the control of the MSC. Handover decisions can also be made in a centralized manner in the MSC. If necessary, the BSC gives a handover command to the MS through the BTS. According to the GSM recommendations, the rate at which measurement results are reported is at least once a second. In most applications, the MS reports the measurement results at intervals of 480 ms.
European Patent Application No. 0 589 278 discloses a microcell/macrocell system. This system comprises measuring the time a mobile station moving in a macro cell remains within the coverage area of a microcell included in the macrocell. The system prevents fast moving mobile stations from being handed over to a microcell base station. Handover from the macrocell to a microcell is performed only if the mobile station is still within the coverage area of a microcell after a predetermined time from its arrival in the microcell area. If the mobile station has already left the coverage area of the microcell during this time, handover from the macrocell to the microcell will not be performed. The problem with this solution is that a handover from a macrocell to a microcell is made on the basis of an instantaneous location of the mobile station. The handover is affected only by the arrival of the mobile station in the coverage area of the microcell and by the location of the mobile station after a predetermined time from its arrival in the area. During this time, a fast moving mobile station may have left the area of the microcell and returned there, in which case the system performs a handover from the macrocell to the microcell. According to an embodiment of the system, the measurement of time is stopped if the mobile station leaves the microcell area and started again from the beginning when the mobile station returns to the microcell area. The problem with this embodiment is that the solution is based on instantaneous measurement results, which do not always fully correspond to the reality. The timer will be reset, for example, if the mobile station moves outside the coverage area of a microcell for a short while, or if a measurement result is erroneous/missing.